During the drilling of oil and/or gas wells, a drill bit at the end of a rotating drill string, or at the end of a drill motor, is used to penetrate through geologic formations. During this operation, drilling mud is circulated through the drill string, out of the bit, and returned to the surface via the annular space between the drill pipe and the formation. Among other functions, the drilling mud provides a washing action to remove the formation cuttings from the wellbore. The mud returns to the surface along with entrained drill cuttings and typically flows through “shale shakers,” desanders, desilters, hydrocyclones, centrifuges, and/or other known devices to separate the cuttings from the mud. The shale shaker(s), which typically sit above the mud storage area, essentially are screens that are used to separate the drill cuttings from the drilling mud. The drilling mud falls through the screens by gravity and the cuttings pass over the end of the screens.
Where drilling is offshore, the disposal of the drill cuttings after separation from the drilling mud can present a problem. The most economical way to dispose of the cuttings would be to simply discharge the cuttings back into the surrounding water. However, the cuttings may contain environmentally damaging “free hydrocarbons,” defined herein as hydrocarbons derived either from the drilling mud, from the formation, or both. The potential for environmental contamination could be alleviated by transporting the cuttings to a disposal facility onshore; however, this would increase the cost of the drilling operation considerably, and would not necessarily improve the environmental performance of the drilling operation.
A typical approach to resolve the problem has been to minimize the toxicity of the base fluids used to make drilling muds, and more recently, to use base fluids which are more biodegradable. Unfortunately, this approach fails to prevent one type of damage that free hydrocarbons can inflict on a marine environment.
Free hydrocarbons are known to organically enrich marine sediment, which eventually causes oxygen depletion and destruction of the environment surrounding the depleted sediment. As with any other organic matter, hydrocarbons tend to break down or decompose in the presence of oxygen, forming carbon dioxide and water. Oxygen is a limiting resource for this reaction. Marine sediment typically has an oxygen content of only from about 2 to about 8 mg per liter of marine sediment. When drill cuttings containing a high concentration of hydrocarbons are discharged into marine waters and reach the sea floor, the oxygen available in the marine sediment rapidly is used to decompose the hydrocarbons. The resulting oxygen depletion very rapidly causes the marine sediment to become anoxic, creating an environment in which most benthic organisms cannot exist.
The potential for environmental damage could be reduced by treating the cuttings in situ before discharging the cuttings into marine waters. Methods are need for treating marine cuttings, preferably in situ, to reduce the quantity of hydrocarbons that will be accessible upon discharge to organically enrich marine sediment.